[clang] Instantiate the initializer of variable templates eagerly

[akemimadoka]

Summary

This patch fixes the bug shown in https://godbolt.org/z/d4a1Kz7r8

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

 /// variable instantiation.
 void Sema::BuildVariableInstantiation(
     VarDecl *NewVar, VarDecl *OldVar,
     const MultiLevelTemplateArgumentList &TemplateArgs,
     LateInstantiatedAttrVec *LateAttrs, DeclContext *Owner,
     LocalInstantiationScope *StartingScope,
     bool InstantiatingVarTemplate,
     VarTemplateSpecializationDecl *PrevDeclForVarTemplateSpecialization) {
-  // Instantiating a partial specialization to produce a partial
-  // specialization.
-  bool InstantiatingVarTemplatePartialSpec =
-      isa<VarTemplatePartialSpecializationDecl>(OldVar) &&
-      isa<VarTemplatePartialSpecializationDecl>(NewVar);
   // Instantiating from a variable template (or partial specialization) to
   // produce a variable template specialization.
   bool InstantiatingSpecFromTemplate =
       isa<VarTemplateSpecializationDecl>(NewVar) &&
       (OldVar->getDescribedVarTemplate() ||
        isa<VarTemplatePartialSpecializationDecl>(OldVar));
 
   // If we are instantiating a local extern declaration, the
if (OldVTSD->getSpecializationKind() == TSK_ExplicitSpecialization &&
           TSK_ExplicitSpecialization);
   }
 
   // Forward the mangling number from the template to the instantiated decl.
   Context.setManglingNumber(NewVar, Context.getManglingNumber(OldVar));
   Context.setStaticLocalNumber(NewVar, Context.getStaticLocalNumber(OldVar));
 
   // Figure out whether to eagerly instantiate the initializer.
-  if (InstantiatingVarTemplate || InstantiatingVarTemplatePartialSpec) {
-    // We're producing a template. Don't instantiate the initializer yet.
-  } else if (NewVar->getType()->isUndeducedType()) {
+  if (NewVar->getType()->isUndeducedType()) {
     // We need the type to complete the declaration of the variable.
     InstantiateVariableInitializer(NewVar, OldVar, TemplateArgs);
   } else if (InstantiatingSpecFromTemplate ||
              (OldVar->isInline() && OldVar->isThisDeclarationADefinition() &&
               !NewVar->isThisDeclarationADefinition())) {
     // Delay instantiation of the initializer for variable template
     // specializations or inline static data members until a definition of the
     // variable is needed.